Process for the production of trioxane

ABSTRACT

The present invention relates to a process for producing cyclic acetal comprising i) preparing a liquid reaction mixture comprising a) formaldehyde source, b) an aprotic compound and c) a catalyst; and ii) converting the formaldehyde source into cyclic acetals.

This present application claims priority to PCT International PatentApplication No, PCT/EP2012/073539 having a filing date of Nov. 23, 2012,and which claims filing benefit to European Patent Application No.11190586.5 filed on Nov. 24, 2011, European Patent Application No,11190574.1 filed on Nov. 24, 2011, and European Patent Application No,11190567.5 filed on Nov. 24, 2011, which are all hereby incorporated byreference in their entirety.

The present invention relates to a process for producing cyclic acetalcomprising preparing a liquid reaction mixture comprising a formaldehydesource, an aprotic compound and a catalyst and converting theformaldehyde source in the reaction mixture to cyclic acetals. Further,the invention relates to a liquid reaction mixture.

1,3,5-Trioxane (hereinafter “trioxane”) is the cyclic trimer offormaldehyde. Trioxane is mainly used as a starting material for themanufacturing of polyoxymethylenes (POM) which is a high performancepolymer having desirable and exceptional properties in terms ofmechanical, chemical and temperature stability. Polyoxymethylenepolymers are available as homo- and copolymers.

As the polyoxymethylene market is growing there is a desire on the sideof the trioxane producers to expand their production capacities in orderto satisfy the trioxane demand on a competitive basis. The majortechnical process for the production of trioxane is the conversion ofaqueous formaldehyde solutions in the presence of concentrated sulfuricacid as a catalyst. The process for the production of trioxane known inthe prior art is complex and comprises an extraction step whichnecessitates tedious solvent recovery steps. Furthermore, the processknown in the prior art is time and energy consuming and leads to a lowdegree of conversion of the formaldehyde source into the desired cyclicacetals (final conversion of less than 10% in the liquid reactionmixture). Furthermore, the amount of side products formed by the processis high.

Technically, the process for the production of trioxane in a liquidsystem is generally the conversion of an aqueous formaldehyde solutionin the presence of sulfuric acid or other homogeneous or heterogeneouscatalysts. However, said technical process has various draw backs.

Under the reaction conditions several side reactions do occur such asthe disproportionation of the formaldehyde to formic acid and methanol(Cannizzaro reaction). The formed acid and methanol may further react tomethyl formiate. Further, the work up procedure and the separation ofthe cyclic acetals, in particular the trioxane, is very time and energyconsuming, complex and cost intensive. A typical process for theproduction of trioxane starts with an aqueous formaldehyde solutionwhich is concentrated by distillation in a first step in order to removethe volume of water. Subsequently, the concentrated formaldehydesolution is fed into a reactor and converted into trioxane in thepresence of a catalyst. The trioxane is separated from the reactionmixture by distillation. However, since the trioxane forms an azeotropewith the water contained in aqueous medium a subsequent extraction stepand a further distillation step to remove the extracting solvent isnecessary. A characteristic of this process is the high energyconsumption for evaporating water which is introduced into the processby the feed stock streams.

There are various proposals for preparing trioxane from formaldehyde bygas-phase trimerization. U.S. Pat. No. 5,508,448 discloses a process forthe preparation of trioxane from formaldehyde in the gas phase whichprocess comprises contacting the formaldehyde with a solid catalystcomprising vanadyl hydrogenphosphate hemihydrates in the gas phase.

However, the gas phase processes generally lead to a low conversion ofthe formaldehyde source into the cyclic acetal. Furthermore, gasreactions require expensive reaction equipment such as pressureresistant vessels and, above all, the reactions are difficult tocontrol.

Thus, the methods for the production of trioxane known in the prior artrequire several costly separation steps and are less efficient.

It was an object of the present invention to provide a process for theproduction of cyclic acetals which is more efficient, leads to a higherfinal conversion and produces cyclic acetals with less side products.Further, it was an object of the invention to provide a process for theproduction of cyclic acetals in a liquid system wherein the energyconsumption is reduced and the separation of the cyclic acetals is lesscomplex.

It has been surprisingly found that the problems associated with themethods disclosed in the prior art can be overcome by forming trioxaneand other cyclic acetals derived from formaldehyde in the presence of anaprotic compound. Further, it has been found that the conversion from aformaldehyde source to cyclic acetals such as trioxane can besignificantly increased.

Accordingly, in a first embodiment the present invention is directed toa process for producing cyclic acetal comprising

-   i) preparing a liquid reaction mixture comprising    -   a) a formaldehyde source,    -   b) an aprotic compound and    -   c) a catalyst; and-   ii) converting the formaldehyde source into cyclic acetals.

A further embodiment of the present invention is a process for producingcyclic acetal comprising reacting a formaldehyde source in the presenceof a catalyst wherein the reaction is carried out in a liquid mediumcomprising an aprotic compound.

An alternative embodiment of the present invention is a process forproducing cyclic acetal from a formaldehyde source in the presence of acatalyst and a liquid medium comprising an aprotic compound.

A further embodiment of the present invention is a liquid reactionmixture comprising

-   a) a formaldehyde source,-   b) an aprotic compound and-   c) a catalyst.

According to a preferred embodiment of the invention the liquid mediumis the aprotic compound.

Thus, further embodiments of the present invention are a process forproducing cyclic acetal comprising reacting a formaldehyde source in thepresence of a catalyst wherein the reaction is carried out in a liquidaprotic compound or, phrased differently, a process for producing cyclicacetal from a formaldehyde source in the presence of a catalyst and aliquid aprotic compound.

A further alternative embodiment is a process for producing cyclicacetal comprising

-   i) preparing a liquid mixture (A) comprising    -   a) a formaldehyde source and    -   b) an aprotic compound; and-   ii) adding a catalyst to the liquid mixture (A); and-   iii) converting the formaldehyde source into cyclic acetals.

A further embodiment is a process for producing cyclic acetal comprising

-   i) preparing a liquid mixture (A) comprising    -   a) a formaldehyde source and    -   b) an aprotic compound;-   ii) contacting the liquid mixture (A) with a catalyst; and-   iii) converting the formaldehyde source to cyclic acetal.

The term “liquid” used in the present invention in conjunction with theaprotic compound, the medium, the mixture (A) and the reaction mixturerefers to the reaction conditions. Under the reaction conditions theliquid system in which the reaction of the formaldehyde source to thecyclic acetal is carried out must be liquid.

An advantage of the present invention is that the conversion of theformaldehyde source is carried out in a liquid system, e.g., a liquidreaction mixture or a liquid medium or a liquid mixture (A). However,even though it is advantageous the components of the reaction mixture orthe liquid mixture (A) must not necessarily completely be dissolved.Thus the reaction mixture or the liquid mixture (A) may also comprisesolids or molten components which are not dissolved.

The formaldehyde source reacts (converts) in the presence of a catalyst.Usually, cationic catalysts, such as Bronsted acids or Lewis acids,accelerate the conversion of the formaldehyde source to the desiredcyclic acetals.

The catalyst is a catalyst for the conversion (reaction) of aformaldehyde source into cyclic acetals, in particular into trioxaneand/or tetroxane.

The methods of the present invention refer to the production of cyclicacetals. Cyclic acetals within the meaning of the present inventionrelate to cyclic acetals derived from formaldehyde. Typicalrepresentatives are showing the following formula:

-   -   wherein a is an integer ranging from 1 to 3.

Preferably, the cyclic acetals produced by the process of the presentinvention are trioxane (a=1) and/or tetroxane (a=2). Trioxane andTetroxane usually form the major part (at least 80 wt.-%, preferably atleast 90 wt.-%) of the cyclic acetals formed by the process of thepresent invention.

The weight ratio of trioxane to tetroxane varies with the catalyst used.Typically, the weight ratio of trioxane to tetroxane ranges from about3:1 to about 40:1, preferably about 4:1 to about 20:1.

An essential feature of the process and the reaction mixture and theliquid mixture (A) of the present invention is the aprotic compound.Contrary to protic compounds such as acids, alcohols and water havingprotons which can be removed relatively easy from the hetero atoms,aprotic compounds preferably have only hydrogen atoms which are linkedto carbon atoms (F. A. Carey, R. J. Lundberg, Organische Chemie, VerlagVCH, 1995, page 224). Generally, aprotic compounds do not containhydrogen atoms which can dissociate i.e., form protons under thereaction conditions.

Advantageously, the aprotic compound does not essentially deactivate thecatalyst. Generally, the catalysts used for the formation of cyclicacetals from a formaldehyde source are cationic catalysts, such asBronsted acids or Lewis acids. Preferably, under the reaction conditionsthe aprotic compound does essentially not deactivate the catalyst usedin the process of the present invention. Aprotic solvents such asdimethylformamide (DMF), dimethylacetamide (DMAC) or N-methylpyrrolidone(NMP) are too basic and therefore may deactivate the catalyst and, as aconsequence, said solvents are less suitable. According to a preferredembodiment of the present invention the liquid reaction mixture isessentially free of amides, preferably essentially free of acylic orcyclic amides. Essentially free means that the amides may be present inan amount of less than about 5 wt.-%, preferably less than about 2wt.-%, more preferably less than 0.5 wt.-%, especially less than about0.01 wt.-% and, in particular, less than 0.001 wt.-% or about 0 wt.-%,wherein the weight is based on the total weight of the liquid reactionmixture. Within the meaning of the present invention the aproticcompound does not deactivate the catalyst if under the reactionconditions less than about 95%, preferably less than about 50%, morepreferably less than about 10%, of the Bronsted acid catalyst usedprotonates the aprotic compound. In case a Lewis acid catalyst is usedthe aprotic compound does not deactivate the catalyst if under thereaction conditions less than about 90 wt-%, preferably less than about50 wt.-%, more preferably less than about 10 wt-% of the Lewis acidcatalyst forms a complex with the aprotic compound.

The degree of protonation and complex formation can be determined by NMRspectroscopy such as ¹H or ¹³C-NMR. The degree of protonation andcomplex formation is determined at 25° C., preferably in d₆-DMSO.

The deactivation of the catalyst can also be determined in the followingmanner:

-   -   10 g of commercially available paraformaldehyde (95 wt %) is        dissolved in 100 g of sulfolane at a temperature sufficient to        dissolve the paraformaldehyde in such a way that no gaseous        formaldehyde can escape. The clear solution is kept at 90° C.        and 0.1 wt % of triflic acid is added. The rate of the formation        of trioxane is measured (by measuring the concentration of        trioxane as a function of time).

The same experiment is repeated, except that 10 g of the sulfolane arereplaced by 10 g of the aprotic compound to be tested. If the rate oftrioxane formation is still greater than about 1%, preferably greaterthan about 5%, more preferably greater than about 10%, of the rate ofthe initial experiment then it is concluded that the aprotic compound inquestion does not deactivate the catalyst (even though it may reduce itsactivity).

The aprotic compound should not be too basic in order to avoiddeactivation of the catalysts. On the other hand the aprotic compoundpreferably does not chemically react with the formaldehyde source underthe reaction conditions.

Preferably, under the reaction conditions the aprotic compound shouldnot react chemically with the formaldehyde source or the cyclic acetalobtained by the process of the invention. Compounds like water andalcohols are not suitable as they react with formaldehyde. Within themeaning of the present invention an aprotic compound does not chemicallyreact with the formaldehyde source when it meets the following testcriteria:

5 g of commercially available paraformaldehyde (95 wt.-%) is added to100 g of the aprotic compound containing 0.1 wt.-%trifluoromethanesulfonic acid and heated at 120° C. for 1 hour withstirring in a closed vessel so that no gaseous formaldehyde can escape.If less than about 1 wt.-%, preferably less than about 0.5 wt.-%, morepreferably less than about 0.1 wt.-% and most preferably less than about0.01 wt.-% of the aprotic compound has chemically reacted, then theaprotic compound is considered not to have reacted with the formaldehydesource.

Further, under the acidic reaction conditions the aprotic compoundshould be essentially stable. Therefore, aliphatic ethers or acetals areless suitable as aprotic compounds. The aprotic compound is consideredstable under acidic conditions within the meaning of the presentinvention if the aprotic compound meets the following test conditions:

100 g of the aprotic compound to be tested containing 0.5% by weight(wt.-%) trifluoromethanesulfonic acid is heated at 120° C. for 1 hour.If less than about 0.5 wt.-%, preferably less than about 0.05 wt.-%,more preferably less than about 0.01 wt.-% and most preferably less thanabout 0.001 wt.-% of the aprotic compound has chemically reacted, thenthe aprotic compound is considered to be stable under acidic conditions.

According to a preferred embodiment of the present invention the aproticcompound is liquid under the reaction conditions. Therefore, the aproticcompound may have a melting point of about 180° C. or less, preferablyabout 150° C. or less, more preferably about 120° C. or less, especiallyabout 60° C. or less.

For practical reasons it is advantageous to use an aprotic compoundwhich has a melting point in the order of preference (the lower themelting point the more preferred) of below about 50° C., below about 40°C. and below about 30° C. and below about 20° C. Especially, aproticcompounds which are liquid at about 25 or about 30° C. are suitablesince they can easily transported by pumps within the production plant.

Further, the aprotic compound may have a boiling point of about 120° C.or higher, preferably about 140° C. or higher, more preferably about160° C. or higher, especially about 180° C. or higher, determined at 1bar. The higher the boiling point the better the cyclic acetals,especially trioxane and/or tetroxane formed by the process of thepresent invention can be separated by distillation. Therefore, accordingto an especially preferred embodiment of the present invention theboiling point of the aprotic compound is at least about 20° C. higherthan the boiling point of the cyclic acetal formed, in particular atleast about 20° C. higher than the boiling point of trioxane and/ortetroxane.

Additionally, aprotic compounds are preferred which do not form anazeotrope with the cyclic acetal, especially do not form an azeotropewith trioxane.

In a preferred embodiment of the present invention the reaction mixturecomprises at least about 20 wt.-%, preferably at least about 40 wt.-%,more preferably at least about 60 wt.-%, most preferably at least about80 wt.-% and especially at least about 90 wt.-% of the aproticcompound(s), wherein the weight is based on the total weight of thereaction mixture. The liquid medium or the reaction mixture or theliquid mixture (A) may comprise one or more aprotic compound(s).

In a preferred embodiment the liquid medium is essentially consisting ofthe aprotic compound. Essentially consisting of means that the liquidmedium comprises at least about 95 wt.-%, preferably at least about 98wt.-%, more preferably at least about 99 wt.-%, especially at leastabout 99.5 wt.-%, in particular at least about 99.9 wt.-% of the aproticcompound(s). In a further embodiment of the invention the liquid mediumis the aprotic compound, i.e., the liquid medium is consisting of theaprotic compound.

It has been found that liquid aprotic compounds which at least partlydissolve the formaldehyde source lead to excellent results in terms ofconversion of the formaldehyde source into the desired cyclic acetals.

Therefore, aprotic compounds are preferred which at least partlydissolve the formaldehyde source under the reaction conditions.Preferred are aprotic compounds which dissolve paraformaldehyde (98wt.-% formaldehyde, 2 wt.-% water) [can also be expressed as Pn=moles offormaldehyde/moles of water=(98/30)/(2/8)=approx. 29] at the reactiontemperature in an amount of at least about 0.1 wt.-%, wherein the weightis based on the total weight of the solution.

Further, preferably the aprotic compound dissolves paraformaldehyde (98wt.-% formaldehyde, 2 wt.-% water; Pn=approx. 29) at 120° C. in anamount of at least about 1 wt.-%, preferably at least about 5 wt.-% andmore preferably at least about 10 wt.-%, wherein the weight is based onthe total weight of the solution.

The aprotic compound used in the process of the invention or thereaction mixture or the liquid mixture (A) of the present invention ispreferably a polar aprotic compound. Polar aprotic solvents are muchmore suitable to dissolve the formaldehyde source. Unpolar aproticcompounds such as unsubstituted hydrocarbons (e.g. cyclic hydrocarbonssuch as cyclohexane, or alicyclic hydrocarbons such as hexane, octane,decane, etc.) or unsubstituted unsaturated hydrocarbons or unsubstitutedaromatic compounds are less suitable. Therefore, according to apreferred embodiment the aprotic compound is not an unsubstitutedhydrocarbon or unsubstituted unsaturated hydrocarbon or unsubstitutedaromatic compound. Further, preferably the reaction mixture comprisesunsubstituted hydrocarbons and/or unsubstituted unsaturated hydrocarbonsand/or unsubstituted aromatic compounds in an amount of less than about50 wt.-%, more preferably less than about 25 wt.-%, further preferablyless than about 10 wt.-%, especially less than about 5 wt.-%, e.g. lessthan about 1 wt.-% or about 0 wt.-%.

Polar aprotic compounds are especially preferred. According to apreferred embodiment of the invention the aprotic compound has arelative static permittivity of more than about 15, preferably more thanabout 20, more preferably of more than about 25, especially of more thanabout 30, determined at 25° C.

The relative static permittivity, ∈_(r), can be measured for staticelectric fields as follows: first the capacitance of a test capacitorC₀, is measured with vacuum between its plates. Then, using the samecapacitor and distance between its plates the capacitance C_(x) with anaprotic compound between the plates is measured. The relative dielectricconstant can be then calculated as

$ɛ_{r} = {\frac{C_{x}}{C_{0}}.}$

Preferred are aprotic compounds which dissolve the formaldehyde source.

According to a preferred embodiment the formaldehyde source is at leastpartially, preferably at least about 80 wt.-%, more preferably at leastabout 95 wt.-%, especially completely, in solution in the reactionmixture or liquid mixture (A).

Therefore the process of the invention is preferably carried out inmanner wherein the formaldehyde source is completely dissolved in theliquid medium or reaction mixture or liquid mixture (A).

Therefore, according to a preferred embodiment the formaldehyde sourceand the aprotic compound form a homogenous phase under the reactionconditions.

Suitable aprotic compounds are selected from the group consisting oforganic sulfoxides, organic sulfones, organic sulfonate ester, nitrilegroup containing organic compounds, halogenated aromatic compounds,nitro group containing aromatic compounds and mixtures thereof.

According to a preferred embodiment the aprotic compound is selectedfrom sulfur containing organic compounds.

Further, the aprotic compound is preferably selected from the groupconsisting of cyclic or alicyclic organic sulfoxides, alicyclic orcyclic sulfones, organic mono- or di-nitrile compounds, nitrobenzene andmixtures thereof.

Excellent results can be achieved by aprotic compounds as represented bythe following formula (I):

-   -   wherein    -   n is an integer ranging from 1 to 6, preferably 2 or 3, and    -   wherein the ring carbon atoms may optionally be substituted by        one or more substituents, preferably selected from C₁-C₈-alkyl        which may be branched or unbranched.

According to the most preferred embodiment the aprotic compound issulfolane (tetrahydrothiophene-1,1-dioxide).

Sulfolane is an excellent solvent for the formaldehyde source, it isstable under acidic conditions, it does not deactivate the catalysts andit does not form an azeotrope with trioxane.

Unless indicated otherwise the expression “reaction mixture” refers tothe mixture which is used for the reaction of the formaldehyde source tothe cyclic acetals. The concentrations and amounts of the individualcomponents of the reaction mixture refer to the concentrations andamounts at the beginning of the reaction. In other words the reactionmixture is defined by the amounts of its starting materials, i.e. theamounts of initial components.

Likewise the amounts defined for the “liquid mixture (A)” refer to theamounts of the components at the beginning of the reaction, i.e. priorto the reaction.

The formaldehyde source reacts to the cyclic acetals and, as aconsequence, the concentration of the formaldehyde source decreaseswhile the concentration of the cyclic acetals increases.

At the beginning of the reaction a typical reaction mixture of theinvention comprises

-   -   a formaldehyde source,

-   a) a catalyst and

-   b) sulfolane.

Further, an especially preferred embodiment of the present invention isa process for producing cyclic acetal comprising reacting a formaldehydesource in the presence of a catalyst wherein the reaction is carried outin sulfolane or a process for producing cyclic acetal from aformaldehyde source in the presence of a catalyst and sulfolane.

A further preferred aprotic compound is represented by formula (II):

-   -   wherein R¹ and R² are independently selected from C₁-C₈-alkyl        which may be branched or unbranched, preferably wherein R¹ and        R² independently represent methyl or ethyl. Especially preferred        is dimethyl sulfone.

According to a further preferred embodiment the aprotic compound isrepresented by formula (III):

-   -   wherein    -   n is an integer ranging from 1 to 6, preferably 2 or 3, and    -   wherein the ring carbon atoms may optionally be substituted by        one or more substituents, preferably selected from C₁-C₈-alkyl        which may be branched or unbranched.

Suitable aprotic compounds are also represented by formula (IV):

-   -   wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl        which may be branched or unbranched, preferably wherein R¹ and        R² independently represent methyl or ethyl.

Especially preferred is dimethyl sulfoxide.

Suitable aprotic compounds may be selected from aliphatic dinitriles,preferably adiponitrile.

The reaction mixture typically comprises the aprotic compound in anamount ranging from about 20 to about 99.85 wt.-%, preferably from about30 to about 99.5 wt.-% or about 30 to about 98 wt.-%, more preferablyfrom about 40 to about 99 wt.-%, further preferably from about 60 toabout 98 wt.-%, especially from about 80 to about 97 wt.-%, based on thetotal weight of the reaction mixture

Further, the reaction mixture specifically comprises the aproticcompound in an amount ranging from 25 to 90 wt.-%, further ranging from25 to 75 wt.-% and in particular from 30 to 65 wt.-%, based on the totalweight of the reaction mixture.

The process of the invention is carried out in the presence of acatalyst for the conversion of the formaldehyde source into cyclicacetals. Suitable catalysts are any components which accelerate theconversion of the formaldehyde source to the cyclic acetals.

The catalyst is a catalyst for the conversion (reaction) of aformaldehyde source into cyclic acetals, preferably into trioxane and/ortetroxane.

Usually, cationic catalysts can be used for the process of theinvention. The formation of cyclic acetals can be heterogeneously orhomogenously catalysed. In case the catalysis is heterogeneous theliquid mixture comprising the formaldehyde source and the aproticcompound is contacted with the solid catalyst or an immiscible liquidcatalyst. A typical liquid immiscible catalyst is a liquid acidic ionexchange resin. Solid catalyst means that the catalyst is at leastpartly, preferably completely in solid form under the reactionconditions. Typical solid catalysts which may be used for the process ofthe present invention are acid ion-exchange material, Lewis acids and/orBronsted acids fixed on a solid support, wherein the support may be aninorganic material such as SiO₂ or organic material such as organicpolymers.

However, preferred is a homogenous catalysis wherein the catalyst isdissolved in the reaction mixture.

Preferred catalysts are selected from the group consisting of Bronstedacids and Lewis acids. The catalyst is preferably selected from thegroup consisting of trifluoromethanesulfonic acid, perchloric acid,methanesulfonic acid, toluenesulfonic acid and sulfuric acid, orderivatives thereof such as anhydrides or esters or any otherderivatives that generate the corresponding acid under the reactionconditions. Lewis acids like boron trifluoride, arsenic pentafluoridecan also be used. It is also possible to use mixtures of all theindividual catalysts mentioned above.

The catalyst is typically used in an amount ranging from about 0.001 toabout 15 wt %, preferably about 0.01 to about 5 wt % or about 0.01 toabout 10 wt.-%, more preferably from about 0.05 to about 2 wt % and mostpreferably from about 0.05 to about 0.5 wt %, based on the total weightof the reaction mixture.

The formaldehyde source used in the process and reaction mixture andliquid mixture (A) of the present invention can in principle be anycompound which can generate formaldehyde or which is formaldehyde or anoligomer or (co)-polymer thereof.

According to a preferred embodiment the formaldehyde source is gaseousformaldehyde.

Gaseous formaldehyde typically comprises traces of water. According to apreferred embodiment the water content is less than about 5 wt.-%,preferably less than about 2 wt.-%, more preferably less than about 1wt.-%, especially less than about 0.5 wt.-%, wherein the weight is basedon the total weight of the sum of the formaldehyde source and the water.

A further preferred formaldehyde source is paraformaldehyde.

Preferably, the paraformaldehyde used has a water content of less thanabout 5 wt.-%, preferably less than about 2 wt.-%, more preferably lessthan about 1 wt.-%, especially less than about 0.5 wt.-%, wherein theweight is based on the total weight of the sum of the formaldehydesource and water.

Another preferred formaldehyde source comprises polyoxymethylene homo-and/or copolymers, preferably with a number average molecular weight(Mn) of more than 2000 Dalton.

The molar mass is determined by GPC (gel permeation chromatography):

-   -   Eluent: hexafluoroisopropanol+0.05% of trifluoroacetic acid        potassium salt    -   Column temperature: 40° C.    -   Flow rate: 0.5 ml/min    -   Detector: differential refractometer Agilent G1362A.

The calibration was effected using PMMA standards having a narrowdistribution from PSS, with molecular weights of M=505 to M=2 740000.Elution ranges outside this interval were estimated by extrapolation.

A further preferred formaldehyde source is formaldehyde which may bepresent in an aqueous solution. The formaldehyde content of the aqueousformaldehyde solution is preferably ranging from about 60 to about 90wt.-%, more preferably ranging from about 65 to about 85 wt.-%, based onthe total weight of the aqueous formaldehyde solution.

The process of the invention can also be used to change the ratio ofcyclic acetals derived from formaldehyde. Therefore, the formaldehydesource can also comprise cyclic acetals selected from the groupconsisting of trioxane, tetroxane and cyclic oligomers derived fromformaldehyde.

Of course, any mixtures of the above-mentioned formaldehyde sources canalso be used.

Preferably, the reaction mixture comprises the formaldehyde source in anamount ranging from about 0.1 to about 80 wt % or about 1 to less thanabout 80 wt.-%, more preferably from about 5 to about 75 wt %, furtherpreferably ranging from about 10 to about 70 wt % and most preferredranging from about 20 to about 70 wt %, especially ranging from 30 to 60wt.-% based on the total weight of the reaction mixture.

According to a preferred embodiment the weight ratio of formaldehydesource to aprotic compound is ranging from about 1:1000 to about 4:1,preferably about 1:600 to about 3:1, more preferably about 1:400 toabout 2:1, further preferably about 1:200 to about 1:1, especiallypreferably about 1:100 to about 1:2, particularly about 1:50 to about1:3, for example about 1:20 to about 1:6 or about 1:15 to about 1:8.

It has been found that protic compounds in the reaction mixture decreasethe degree of conversion. Therefore, it is desired that the amount ofprotic compounds is as low as possible.

According to a preferred embodiment of the present invention the amountof protic compounds, in particular the amount of water and alcohols, isless than about 20 wt.-%, preferably less than about 15 wt.-%, morepreferably less than about 10 wt.-%, further preferably less than about5 wt.-%, especially preferably less than about 2 wt.-%, in particularless than about 1 wt.-%, for example less than about 0.5 wt.-%, based onthe total amount of the liquid reaction mixture.

According to an especially preferred embodiment of the invention theamount of water in the reaction mixture is less than about 20 wt.-%,preferably less than about 15 wt.-%, more preferably less than about 10wt.-%, further preferably less than about 5 wt.-%, especially preferablyless than about 2 wt.-%, in particular less than about 1 wt.-%, forexample less than about 0.5 wt.-%, based on the total amount of theliquid reaction mixture.

A preferred embodiment of the process of the present invention is aprocess for producing cyclic acetal comprising

-   i) preparing a liquid reaction mixture comprising    -   a) 0.1 to less than 80 wt.-% of a formaldehyde source,    -   b) 20 to 99.85 wt.-% of an aprotic compound and    -   c) 0.001 to 15 wt % of a catalyst; and-   ii) converting the formaldehyde source into cyclic acetals.

An especially preferred embodiment of the present invention is a processfor producing cyclic acetal, preferably trioxane and/or tetroxane,comprising

-   i) preparing a liquid reaction mixture comprising    -   a) 20 to 70 wt.-%, preferably 30 to 60 wt.-%, of a formaldehyde        source, preferably selected from the group consisting of gaseous        formaldehyde, paraformaldehyde, polyoxymethylene homo- and        copolymers, an aqueous formaldehyde solution, trioxane,        tetroxane, cyclic oligomers derived from formaldehyde and        mixtures thereof    -   b) 25 to 75 wt.-%, preferably 30 of 65 wt.-%, of an aprotic        compound, preferably selected from sulfolane, dimethyl        sulfoxide, dimethyl sulfone and nitrobenzene, especially        sulfolane;    -   c) 0.001 to 10 wt % of a catalyst, preferably selected from        Bronsted and Lewis acids; and    -   d) optional less than 20 wt.-% of water; and-   ii) converting the formaldehyde source into cyclic acetals,    preferably trioxane and/or tetroxane.

Typically, the reaction is carried out at a temperature higher thanabout 0° C., preferably ranging from about 0° C. to about 150° C., morepreferably ranging from about 10° C. to about 120° C., furtherpreferably from about 20° C. to about 100° C. and most preferably fromabout 30° C. to about 90° C.

A further advantageous of the process of the present invention is thatthe cyclic acetals can easily be separated from the reaction mixture.The cyclic acetal, especially the trioxane can be separated from thereaction mixture by distillation in a high purity grade. Especially incase aprotic compounds (such as sulfolane) having a boiling point higherthan about 20° C. above the boiling point of the cyclic acetals are usedthe formed cyclic acetals can simply be distilled off. In case sulfolaneis used as the aprotic compound the formed trioxane can be distilled offwithout the formation of an azeotrope of sulfolane with trioxane. Theprocess of the invention can be carried out batch wise or as acontinuous process.

In a preferred embodiment the process is carried out as a continuousprocess wherein the formaldehyde source is continuously fed to theliquid medium comprising the catalyst and wherein the cyclic acetals,e.g. the trioxane, is continuously separated by separation methods suchas distillation.

The process of the invention leads to an extremely high conversion ofthe formaldehyde source to the desired cyclic acetals. Conversionshigher than 10 for the production of trioxane in liquid media are notknown in the prior art.

According to a preferred embodiment the final conversion of theformaldehyde source to the cyclic acetal is greater than 10%, based oninitial formaldehyde source.

The final conversion refers to the conversion of the formaldehyde sourceinto the cyclic acetals in the liquid system. The final conversioncorresponds to the maximum conversion achieved in the liquid system.

The final conversion of the formaldehyde source to the cyclic acetalscan be calculated by dividing the amount of cyclic acetals (expressed inwt.-%, based on the total weight of the reaction mixture) in thereaction mixture at the end of the reaction divided by the amount offormaldehyde source (expressed in wt.-%, based on the total weight ofthe reaction mixture) at the beginning of the reaction at t=0.

For example the final conversion of the formaldehyde source to trioxanecan be calculated as:Final conversion=(amount of trioxane in the reaction mixture expressedin weight-% at the end of the reaction) (amount of formaldehyde sourcein the reaction mixture expressed in weight-% at t=0 [initial amount offormaldehyde source in the reaction mixture])

According to a further preferred embodiment of the process of theinvention the conversion which can e.g. be the final conversion of theformaldehyde source into the cyclic acetals, preferably trioxane and/ortetroxane, is higher than 12%, preferably higher than 14%, morepreferably higher than 16%, further preferably higher than 20%,especially higher than 30%, particularly higher than 50%, for examplehigher than 80% or higher than 90%.

-   -   Preferably, at least partially converting the formaldehyde        source into the cyclic acetal means that the conversion of the        formaldehyde source into the cyclic acetals, preferably trioxane        and/or tetroxane, is greater than 10%, based on the initial        formaldehyde source, especially higher than 12%, preferably        higher than 14%, more preferably higher than 16%, further        preferably higher than 20%, especially higher than 30%,        particularly higher than 50%, for example higher than 80% or        higher than 90%.

The liquid reaction mixture of the present invention comprises

-   a) a formaldehyde source,-   b) an aprotic compound and-   c) a catalyst.

The preferred amounts and components a) to c) are described throughoutthe description of the present invention.

Especially preferred is a liquid reaction mixture comprising

-   a) 5 to 70 wt.-%, preferably 20 to 70 wt.-%, more preferably 30 to    60 wt.-%, of a formaldehyde source, preferably selected from the    group consisting of gaseous formaldehyde, paraformaldehyde,    polyoxymethylene homo- and copolymers, an aqueous formaldehyde    solution, trioxane, tetroxane, cyclic oligomers derived from    formaldehyde and mixtures thereof,-   b) 25 to 90 wt.-%, preferably 25 to 75 wt.-%, more preferably 30 of    65 wt.-%, of an aprotic compound, preferably selected from    sulfolane, dimethyl sulfoxide, dimethyl sulfone and nitrobenzene,    especially sulfolane;-   c) 0.001 to 10 wt % of a catalyst, preferably selected from Bronsted    and Lewis acids; and-   d) optional less than 20 wt.-% of water, wherein the amounts are    based on the total weight of the reaction mixture.

A further embodiment of the present invention is a liquid mixture (A)comprising

-   a) a formaldehyde source and-   b) an aprotic compound.

The preferred components a) and b) for the liquid mixture (A) of theinvention are described throughout the description of the presentinvention.

Preferably, the liquid mixture (A) comprises the formaldehyde source inan amount ranging from about 0.1 to about 80 wt.-% or about 1 to lessthan about 80 wt.-%, more preferably from about 5 to about 75 wt.-%,further preferably ranging from about 10 to about 70 wt % and mostpreferred ranging from about 20 to about 70 wt.-%, especially rangingfrom 30 to 60 wt.-% based on the total weight of the liquid mixture (A).

The liquid mixture (A) typically comprises the aprotic compound in anamount ranging from about 20 to about 99.85 wt.-%, preferably from about30 to about 99.5 wt.-% or about 30 to about 98 wt.-%, more preferablyfrom about 40 to about 99 wt.-%, further preferably from about 60 toabout 98 wt.-%, especially from about 80 to about 97 wt.-%, based on thetotal weight of the liquid mixture (A).

Further the reaction mixture specifically comprises the aprotic compoundin an amount ranging from about 25 to about 90 wt.-%, further rangingfrom about 25 to about 75 wt.-% and in particular from about 30 to about65 wt.-%, based on the total weight of the liquid mixture (A).

According to a preferred embodiment of the present invention the amountof protic compounds, in particular the amount of water and alcohols, inthe liquid mixture (A) is less than about 20 wt.-%, preferably less thanabout 15 wt.-%, more preferably less than about 10 wt.-%, furtherpreferably less than about 5 wt.-%, especially preferably less thanabout 2 wt.-%, in particular less than about 1 wt.-%, for example lessthan about 0.5 wt.-%, based on the total amount of the liquid mixture(A).

According to an especially preferred embodiment of the invention theamount of water in the liquid mixture (A) is less than about 20 wt.-%,preferably less than about 15 wt.-%, more preferably less than about 10wt.-%, further preferably less than about 5 wt.-%, especially preferablyless than about 2 wt.-%, in particular less than about 1 wt.-%, forexample less than about 0.5 wt.-%, based on the total amount of theliquid mixture (A).

A further embodiment is a process for producing cyclic acetal comprising

-   i) preparing a liquid mixture (A) comprising    -   a) a formaldehyde source and    -   b) an aprotic compound;-   ii) contacting the liquid mixture (A) with a catalyst; and-   iii) converting the formaldehyde source into cyclic acetal.

According to this embodiment of the present invention a liquid mixture(A) as defined above can be prepared and contacted with a catalyst asdefined above. According to a preferred embodiment the catalyst is asolid catalyst which at least remain partly solid under the reactionconditions. Preferably the catalyst is selected from fixed bed catalyst,acid ion-exchange material and solid support carrying Bronsted and/orLewis acids.

The liquid mixture (A) is preferably comprising

-   a) 5 to 70 wt.-%, preferably 20 to 70 wt.-%, more preferably 30 to    60 wt.-%, of a formaldehyde source, preferably selected from the    group consisting of gaseous formaldehyde, paraformaldehyde,    polyoxymethylene homo- and copolymers, an aqueous formaldehyde    solution, trioxane, tetroxane, cyclic oligomers derived from    formaldehyde and mixtures thereof,-   b) 25 to 90 wt.-%, preferably 25 to 75 wt.-%, more preferably 30 of    65 wt.-%, of an aprotic compound, preferably selected from    sulfolane, dimethyl sulfoxide, dimethyl sulfone and especially    sulfolane;-   c) optional 0.001 to 10 wt % of a catalyst, preferably selected from    Bronsted and Lewis acids; and-   d) optionally less than 20 wt.-% of protic compounds, especially    water, wherein the amounts are based on the total weight of the    liquid mixture (A).

A further embodiment of the present invention is the use of an aproticcompound for the production of cyclic acetals.

The preferred aprotic compounds do not deactivate the catalyst, do notform an azeotrope with trioxane and do have a boiling point of at least20° C. higher than the boiling point of trioxane at 1 bar.

The preferred aprotic compounds are defined throughout the description.Preferably a polar aprotic compound, more preferably selected from thegroup consisting of sulfolane, dimethyl sulfoxide, dimethyl sulfone, andespecially sulfolane, is used for the production of cyclic acetals,preferably trioxane and/or tetroxane.

Preferred embodiments of the present invention refer to:

-   1. A process for producing cyclic acetal comprising-   i) preparing a liquid reaction mixture comprising    -   a) a formaldehyde source,    -   b) an aprotic compound and    -   c) a catalyst; and-   ii) converting the formaldehyde source into cyclic acetals.-   2. A process according to item 1. wherein the aprotic compound is    liquid under the reaction conditions.-   3. A process according to items 1. or 2. wherein the aprotic    compound has a boiling point of 120° C. or higher, preferably    140° C. or higher, more preferably 160° C. or higher, especially    180° C. or higher, determined at 1 bar.-   4. A process according to one or more of the preceding items wherein    the reaction mixture comprises at least 20 wt.-%, preferably at    least 40 wt.-%, more preferably at least 60 wt.-%, most preferably    at least 80 wt.-% and especially at least 90 wt.-% of the aprotic    compound, wherein the weight is based on the total weight of the    reaction mixture.-   5. A process according to one or more of the preceding items wherein    the aprotic compound is selected from the group consisting of    organic sulfoxides, organic sulfones, organic sulfonate ester,    nitrile group containing organic compounds, halogenated aromatic    compounds, nitro group containing aromatic compounds and mixtures    thereof, preferably the aprotic compound is selected from sulfur    containing organic compounds.-   6. A process according to one or more of the preceding items wherein    the aprotic compound is selected from the group consisting of cyclic    or alicyclic organic sulfoxides, alicyclic or cyclic sulfones,    organic mono- or di-nitrile compounds, nitrobenzene and mixtures    thereof.-   7. A process according to one or more of the preceding items wherein    the aprotic compound is represented by formula (I):

wherein

-   n is an integer ranging from 1 to 6, preferably 2 or 3, and-   wherein the ring carbon atoms may optionally be substituted by one    or more substituents, preferably selected from C1-C8-alkyl which may    be branched or unbranched.-   8. Process according to one or more of the preceding items wherein    the aprotic compound is sulfolane.-   9. Process according to one or more of items 1. to 6. wherein the    aprotic compound is represented by formula (II):

-   wherein R¹ and R² are independently selected from C1-C8-alkyl which    may be branched or unbranched, preferably wherein R¹ and R²    independently represent methyl or ethyl, preferably the aprotic    compound is dimethyl sulfone.-   10. Process according to one or more of items 1. to 6. wherein the    aprotic compound is represented by formula (III):

wherein

-   n is an integer ranging from 1 to 6, preferably 2 or 3, and-   wherein the ring carbon atoms may optionally be substituted by one    or more substituents, preferably selected from C1-C8-alkyl which may    be branched or unbranched; or-   the aprotic compound is represented by formula (IV):

wherein R³ and R⁴ are independently selected from C1-C8-alkyl which maybe branched or unbranched, preferably wherein R¹ and R² independentlyrepresent methyl or ethyl; preferably the aprotic compound is dimethylsulfoxide.

-   11. A process according to one or more of the preceding items    wherein the catalyst is selected from the group consisting of    Bronsted acids and Lewis acids.-   12. Process according to one or more of the preceding items wherein    the formaldehyde source is selected from the group consisting of    gaseous formaldehyde, paraformaldehyde, polyoxymethylene homo- and    copolymers, and an aqueous formaldehyde solution and mixtures    thereof.-   13. Process according to one or more of the preceding items wherein    the formaldehyde source and the aprotic compound form a homogenous    phase.-   14. Process according to one or more of the preceding items wherein    the reaction is carried out at a temperature higher than 0° C.,    preferably ranging from 0° C. to 150° C., more preferably ranging    from 10° C. to 120° C., further preferably from 20° C. to 100° C.    and most preferably from 30° C. to 90° C.-   15. Liquid reaction mixture comprising a formaldehyde source, a    catalyst and an aprotic compound.

EXAMPLES Example 1

Anhydrous formaldehyde was prepared by the thermal decomposition ofparaformaldehyde (essay: 96 wt %, from Acros Organics) at a rate of ca.1 g/min at appr. 120° C. and a pressure of 80 mbar. The formaldehyde gaswas absorbed in a absorption column containing 500 g sulfolane (<0.1 wt% water) with 0.1 wt % triflic acid at around 40° C. After 1 hr, thesulfolane in the adsorption column was neutralized with triethylamineand analyzed by GC and sulfite titration. The following composition wasfound:

-   Trioxane: 8.3 wt %-   Tetroxane: 1.1 wt %-   Formaldehyde: 0.6 wt %-   Methyl formate: 0.5 wt %

Final conversion of formaldehyde to trioxane in the reaction mixture:77.5%

Final conversion of formaldehyde to trioxane and tetroxane in thereaction mixture: 88%

Example 2

-   -   500 g of an aqueous 80 wt. % solution of formaldehyde were mixed        with 500 g of sulfolane at 80° C. 40 g of concentrated sulfuric        acid were added and the clear mixture was heated to 100° C. and        kept there for 15 min. Then 50 ml were distilled off at        atmospheric pressure and analyzed:        The distillate contained:

-   32 wt % trioxane

-   0.05 wt % methyl formate

Comparative Example 3

-   -   To 100 g of a 60 wt.-% solution of formaldehyde in water at        100° C. 5 g of sulfuric acid is added. After 15 min ca. 5 g were        distillated off at atmospheric pressure. The trioxane        concentration in the distillate was 22 wt.-%.        This shows that the process of the invention is more effective        and requires less energy to separate the cyclic acetal due to        the higher trioxane concentration in the distillate.

Example 4

-   -   9 g of commercial paraformaldehyde with a water content of ca. 4        wt % (essay: 96 wt % from Acros Organics) were added to 91 g of        sulfolane at 145° C. with stirring. As the paraformaldehyde        dissolves, the temperature decreases to 122° C. The clear        solution was allowed to cool to 100° C. At that temperature 0.3        ml of a 10 wt % solution of triflic acid in sulfolane was added.        After 1 min, the homogeneous solution was allowed to cool to 60°        C., was neutralized with triethylamine and then analyzed. The        following composition was found:

-   Trioxane: 7.0 wt %

-   Tetroxane: 0.6 wt %

-   Formaldehyde: 1 wt %

Example 5

-   -   10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane        content) (TICONA trade name: Hostaform® HS 15) with melt index        of 1.5 g/10 min were dissolved in 90 g of sulfolane at 145° C.        with stirring. The clear solution was added to 20 g sulfolane        (at 120° C.) containing 0.4 ml of a 10 wt % solution of triflic        acid in sulfolane. After the addition was completed, the        homogeneous solution was cooled to 60° C., neutralized with        triethylamine and then analyzed. The following composition was        found:

-   Trioxane: 7.1 wt %

-   Tetroxane: 0.75 wt %

-   Formaldehyde: 0.4 wt %

-   Methylformate: <20 ppm

Example 6

-   -   Example 5 was repeated, except that perchloric acid (70 wt % in        water) was used for triflic acid:    -   10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane        content) (TICONA trade name: Hostaform® HS 15) with melt index        of 1.5 g/10 min were dissolved in 90 g of sulfolane at 145° C.        with stirring. The clear solution was added to 20 g sulfolane        (at 120° C.) containing 1.2 ml of a 2 wt % solution of        perchloric acid (70 wt % in water) in sulfolane. After the        addition was completed, the homogeneous solution was cooled to        60° C., neutralized with triethylamine and then analyzed. The        following composition was found:

-   Trioxane: 7.2 wt %

-   Tetroxane: 0.8 wt %

-   Formaldehyde: 0.3 wt %

-   Methylformate: <20 ppm

Comparative Example 7

-   -   Example 5 was repeated, except that nitrobenzene was used for        sulfolane as a solvent:    -   10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane        content) (TICONA trade name: Hostaform® HS 15) with melt index        of 1.5 g/10 min were dissolved in 90 g of nitrobenzene at        145° C. with stirring. The clear solution was added to 20 g        nitrobenzene (at 120° C.) containing 0.4 ml of a 10 wt %        solution of triflic acid in sulfolane. After the addition was        completed, the homogeneous solution was cooled to 60° C.,        neutralized with triethylamine and then analyzed. The following        composition was found:

-   Trioxane: 6.2 wt %

-   Tetroxane: 0.7 wt %

-   Formaldehyde: 0.7 wt %

-   Methylformate: 0.5 wt %    -   The GC spectrum also showed a new peak with a retention time        beyond that of nitrobenzene, which was not further analyzed but        is believed to be a reaction product of nitrobenzene with        formaldehyde. Thus, nitrobenzene is not stable under reaction        conditions, produces side products (methylformate) and        consequently has a lower yield in trioxane.

Example 8

-   -   Example 5 was repeated, except that a mixture of Dimethylsulfone        (30 g) and Sulfolane (60 g) was used for sulfolane as a solvent:    -   10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane        content) (TICONA trade name: Hostaform® HS 15) with melt index        of 1.5 g/10 min were dissolved in a mixture of Dimethylsulfone        (30 g) and Sulfolane (60 g) at 145° C. with stirring. The clear        solution was added to 20 g sulfolane (at 120° C.) containing 0.4        ml of a 10 wt % solution of triflic acid in sulfolane. After the        addition was completed, the homogeneous solution was cooled to        60° C., neutralized with triethylamine and then analyzed. The        following composition was found:    -   Trioxane: 7.1 wt %    -   Tetroxane: 0.6 wt %    -   Formaldehyde: 0.8 wt %    -   Methylformate: 9.4 ppm

Example 9

-   -   Example 4 was repeated except that strongly acidic ion exchange        resin (Amberlyst 15®, wet form, from DOW CHEMICAL) was used        instead of triflic acid as catalyst.    -   Before use the resin was conditioned to sulfolane (exchange of        water in the pores of the resin by sulfolane)        9 g of commercial paraformaldehyde with a water content of ca. 4        wt % (essay: 96 wt % from Acros Organics) were added to 91 g of        sulfolane at 145° C. with stirring. As the paraformaldehyde        dissolves the temperature decreases to 122° C. The clear        solution was allowed to cool to 100° C. At that temperature 10 g        of Amberlyst 15® was added. After 10 min at 100° C. the reaction        mixture was allowed to cool to 50° C., and no precipitate        formed, indicating the conversion of the paraformaldehyde to        trioxane. The concentration of the trioxane in the reaction        mixture is estimated to be above 6 wt %.

The invention claimed is:
 1. A process for producing a cyclic acetalcomprising: contacting a formaldehyde source with a liquid mediumcomprising a sulfur-containing organic compound and a catalyst, thecatalyst comprising trifluoromethanesulfonic acid, perchloric acid,methanesulfonic acid, toluenesulfonic acid, sulfuric acid, or a solidacid ion-exchange materiel; and at least partially converting theformaldehyde source into a cyclic acetal, wherein the cyclic acetalcomprises trioxane.
 2. A process according to claim 1, wherein thesulfur-containing organic compound has a boiling point of 140° C. orhigher, determined at 1 bar.
 3. A process according to claim 1 whereinhigher than 10% of the formaldehyde source is converted into one or morecyclic acetals during the reaction.
 4. A process according to claim 1wherein the liquid medium comprises at least 20 wt.-%, of thesulfur-containing organic compound.
 5. A process according to claim 1,wherein the sulfur-containing organic compound is selected from thegroup consisting of organic sulfoxides, organic sulfones, organicsulfonate esters, and mixtures thereof.
 6. A process according to claim1 wherein the sulfur-containing organic compound is selected from thegroup consisting of cyclic or alicyclic organic sulfoxides, alicyclic orcyclic sulfones, and mixtures thereof.
 7. A process according to claim 1wherein the sulfur-containing organic compound is represented by formula(I):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,selected from C₁-C₈-alkyl which may be branched or unbranched or by theformula (II):

wherein R¹ and R² are independently selected from C₁-C₈-alkyl which maybe branched or unbranched or by the formula (III):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,selected from C₁-C₈-alkyl which may be branched or unbranched; or by theformula (IV):

wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl which maybe branched or unbranched.
 8. A process according to claim 1 wherein thesulfur-containing organic compound is sulfolane.
 9. A process accordingto claim 1 wherein the reaction produces trioxane and tetroxane.
 10. Aprocess according to claim 1 wherein during the process a reactionmixture includes the formaldehyde source, the sulfur-containing organiccompound, and the catalyst, and wherein the reaction mixture comprisesthe sulfur-containing organic compound in an amount ranging from about25 wt.-% to about 90 wt.-%.
 11. A process according to claim 1, whereinthe catalyst comprises trifluoromethanesulfonic acid, perchloric acid,methanesulfonic acid, toluenesulfonic acid, or sulfuric acid.
 12. Aprocess according to claim 1, wherein the formaldehyde source has awater content of less than about 20 wt.-%.
 13. A process according toclaim 1, wherein the formaldehyde source comprises an aqueousformaldehyde solution.
 14. A process according to claim 13, wherein theaqueous formaldehyde solution contains formaldehyde in an amount fromabout 60 wt. % to about 90 wt. %.
 15. A process according to claim 1,further comprising the step of separating the cyclic acetal from theliquid medium by distillation.
 16. A process according to claim 1,further comprising the step of manufacturing polyoxymethylene from thecyclic acetal.
 17. A process according to claim 1, wherein theformaldehyde source comprises gaseous formaldehyde.
 18. A process forproducing cyclic acetal comprising i) preparing a liquid reactionmixture comprising a) a formaldehyde source, b) an aprotic compoundcomprising a sulfur-containing organic compound and c) a catalyst, thecatalyst comprising trifluoromethanesulfonic acid, perchloric acid,methanesulfonic acid, toluenesulfonic acid, sulfuric acid, or a solidacid ion-exchangepaterial; and ii) converting the formaldehyde sourceinto cyclic acetals; wherein the cyclic acetals comprise trioxane.
 19. Aprocess according to claim 1, wherein the catalyst comprises a solidacid ion-exchange material.
 20. A process according to claim 10, whereinthe reaction mixture includes the formaldehyde source in an amount fromabout 20 wt. % to about 70 wt. %, contains the aprotic compound in anamount from about 25 wt. % to about 75 wt. %, and contains water in anamount less than about 20 wt. %.
 21. A process according to claim 13,wherein during the process a reaction mixture comprises the formaldehydesource, the sulfur containing organic compound, and the catalyst andwherein the reaction mixture contains water in an amount less than about20 wt. %.
 22. A process for producing a cyclic acetal comprising:contacting a formaldehyde source with a liquid medium comprising asulfur-containing organic compound in the presence of a catalyst, thesulfur-containing organic compound comprising an organic sulfoxide, asulfone, or mixtures thereof; and at least partially converting theformaldehyde source into a cyclic acetal, wherein the cyclic acetalcomprises trioxane.
 23. A process as defined in claim 22, wherein thesulfur-containing organic compound comprises sulfolane.